Metastasis is a multifactorial process by which tumor cells escape from the primary tumor disseminate through blood and lymph vessels evade host immune defence and home to specific target organs where they extravasate and re-colonize. Metastatic cells must encounter and cross the basement membrane during the extravasation of the blood vessels. The metastatic process is usually described as a three-step model, wherein metastatic cells must first adhere to the basement membrane, then digest via proteolytic enzymes the basal lamina, and finally migrate through the vessel wall.
Evaluating the metastatic potential of a specific tumor is essential for diagnostic and prognostic purposes, for determining optimal course of treatment as well as for various research purposes such as developing therapeutical methods and medicaments for the treatment of metastasis.
Correlation between various biomolecules and the metastatic tendency of tumor cells has been previously described. There has been a disclosure of association of increased basement membrane invasiveness, which is the first step in the process of metastasis, with the absence of estrogen receptor and expression of vimentin in a human breast cancer cell line (Thompson et al, J. of Cellular Plzysiol., 150:534-544 (1992)). A correlation between the invasive potential of various human breast cancer cell lines and binding and degradation of hyaluronan has also been described (Cultry et al., J. of Cellular Physiol., 160:275-286 (1994)). Metastatic potential of various tumor cell lines has also been associated with amplification in the expression of various proto-oncogenes, such as the HER-2/neu proto-oncogene which is amplified in 25% of human primary breast cancer and its amplification is associated with shorter time to relapse and shorter overall survival (Slamon et al., Science, 244:707-712 (1989)).
Although currently change in the level of some of the above biomolecules, especially amplification of the HER-2/neu proto-oncogenes, is used for prognostic purposes, the prognostic use of expression or amplification of this gene has been quite controversial (Press, M. F. et al, Cancer Res, 53:4960-4970 (1993)). The prognostic factors currently used to assess the breast tumor state are: tumor size, histological grade, steroid hormone receptor status, DNA ploidity, proliferative index, cathepsin D and analysis of growth factor receptors such as epidermal growth factor receptor.
There is a great need today for additional, accurate prognostic factors for tumor cells which will be easy to identify and which are in good correlation to the metastatic tendency of those cells.
The thrombin receptor (ThR) has been cloned and identified as a member of the seven trans-membrane domain super-family of G-protein coupled receptors. It is activated by cleavage of the Arg.sup.41 -Ser.sup.42 residues of the extracellular N-terminus part of the receptor by the protease thrombin. This cleavage exposes the ligand of the thrombin receptor which is an integral part of the receptor itself. Thus, the ThR serves as a classical substrate for protease, rather than the traditional ligand-receptor complex, during the course of cellular activation (Vu, et al., Cell, 64:1057-1068, (1991)). Recently, there has been a report that W256 carcinoma and mouse melanoma tumor cells contain functional thrombin receptors (Wojtukiewicz, et al, Cancer Res., 55:698-704 (1995)), however, only the existence and function of the thrombin receptor has been assessed in this publication and there has been no determination of the level of its expression, nor its connection to the malignancy of the tumor.